Offshore platform structures are moored to ships during the transfer of oil between the platform and a tanker, and also during the transfer of supplies from a supply boat to the platform. Many systems have been developed for mooring and cargo transfer of a tanker, which generally involve the use of hoses to carry oil or other hydrocarbons between the platform and tanker. Such systems also generally include structures that hold the tanker close enough to the platform structure to avoid breaking the hoses while permitting the tanker to freely drift about the platform structure. A different type of mooring approach is required in the mooring of supply boats, from which cargo such as large cartons are to be transferred to the platform structure by large cranes on the structure. Such transfer of cargo from a supply boat requires that it be held at a substantially fixed location and compass heading.
A conventional mooring method employed for supply boats unloading at an offshore platform, requires the stern of the vessel to be moored by two quarter lines to the platform structure, while the engines of the vessel are operated to keep the quarter lines taut. Another method is to use a buoyed cable which is anchored to the seabed, and which the supply boat picks up and attaches to its own anchor cable. The supply boat then backs down to the platform to apply tension to the anchored cable. The reliance upon the supply boat's own engines or anchor system has been found to be hazardous.
An improved supply boat mooring technique involves the use of a mooring buoy which is anchored by a chain to a mooring base at the seabed, and with the top of the buoy being attached by a hawser to the bow of the supply boat. Tension in the hawser causes the buoy to tip, which results in a restoring force that keeps the hawser taut. While this mooring system operates well, it can lead to considerable expense in deep water installations. The secure emplacement of a mooring base at great depths such as over 100 meters, is considerable, and the cost of maintenance where divers have to descend to the mooring base, is also considerable. A mooring system for mooring supply boats to deep water platform structures, which provided reliable holding of the supply boats while avoiding the expense of installing and maintaining mooring bases at the great depths of the seabed, would be of considerable value in reducing the cost of offshore installations.